The winner will only be the fifth person to give the time "at the third stroke", after people dial 123.

Sara Mendes da Costa has been the voice of the speaking clock since winning the last competition in 2007.

Previous voices have included an actor and a London telephone exchange supervisor.

Patricia Simmons was the voice of the speaking clock in the 1960sCredit:
Getty

To celebrate the 80th birthday, BT is donating the original speaking clock machines from 1936 and 1963 to the British Horological Institute (BHI), where they have been on loan and on display for some years.

David Hay, head of BT Heritage, said: "Eighty years ago BT's technology first created the speaking clock and it remains a much loved part of British life today.

"Celebrating in this way demonstrates BT's determination to preserve the heritage of the world's oldest communications company on behalf of the nation."

Some of the famous voices who have been heard as the speaking clock include Garry Barlow, Chris Moyles and Cheryl Cole, who all took part for charity.

The competition is being run in partnership with BBC TV's The One Show, with the winner announced in November.